Abstract: The incubation experiment was conducted to investigate the impacts of rice straw and its biochar product on the amounts of soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) and the mineralization of soil organic nitrogen of a typical dryland soil and a paddy soil in subtropical region of China. The results showed that, after 35 days of incubation, the addition of biochar (derived from rice straw) increased MBC, MBN and nitrogen mineralization by 34.6%, 163.1% and 6.4% respectively in the dryland soil and increased MBN and nitrogen mineralization by 23.0% and 15.1% respectively in paddy soil, compared with the no biochar addition treatment. The addition of rice straw increased MBC and MBN by 90.4% and 203.8% respectively, the nitrogen mineralization was decreased by 22.2% in the dryland soil, and increased MBC, MBN and nitrogen mineralization by 13.4%, 19.9% and 7.3% respectively in the paddy soil, compared with the no straw addition treatment. In conclusion, both additions of biochar and rice straw increased microbial biomass in croplands, particularly with the rice straw. The nitrogen mineralization was also increased by the additions of biochar and rice straw, except for in the dryland soil with the rice straw addition. The contribution of rice straw and its biochar product to the dynamics of soil MBC and MBN and the turnover of soil organic nitrogen needs further investigations to clarify, such as by using the isotope techniques.